# Windowing

An amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.

The FFT analyzer does not operate in a continuous manner, but is instead a batch processing device, taking samples of the time domain signal and transforming them into a frequency domain spectrum. The time interval during which the signal is sampled and recorded is called the window. In order to compensate for certain limitations of the FFT process, the time data in the window are often multiplied by a weighting curve, such as a Hanning or Flattop weighting.

### Transient analysis

**Uniform window**- for general purposes.**Force window**- for short impulses and transients, to improve the signal to noise ratio.**Exponential window**- for transients longer than the record length, transients that do not decay sufficiently within the record length.

### Continuous Signal Analysis

**Uniform window**- should only be used when analysing special sinusoids, whose frequencies coincide with the centre frequencies in the analysis. This is often the situation when pseudo-random types of signal are analysed, or when order tracking is applied.**Hanning window**- is a general purpose window and should be used in most cases, because it has the best overall filter characteristic.**Kaiser-Bessel and Blackman-Harris window**- show very good selectivity and should be used for two-tone seperation of harmonic signals with widely different levels.**Flattop window**- is mainly desiigned for calibration and correct amplitude measurements.

### Frequency Response Measurements

**Force window**- for the excitation signal when an impact hammer is used for excitation.**Exponential window**- for the response signal of lightly damped systems, when an impact hammer is used for excitation.**Hanning window**- for both excitation and response signal when a random excitation signal is used.**Uniform window**- for both excitation and response signals when a pseudo-random excitation signal is used.

**See also: **Blackman Window, Blackman-Harris Window, Cauchy Window, Discrete Fourier Transform, Exponential Window, Fast Fourier Transform, Flattop Window, Force Window, Hamming Window, Hanning Window, Hanning-Poisson Window, Kaiser Bessel Window, Leakage.

**Subjects: ** Noise & Vibration Signal Processing